In recent years, along with an expanded amount of information required by an information apparatus, an image audio apparatus and the like, public attention has been focused on an optical disk as a recording medium, because of its ease in data access, capability of storing a large amount of data, and compactness, wherein an attempt has been made to achieve high-density recorded information on such a recording medium. For example, with respect to a method for achieving a high-density optical disk, an optical recording medium has been proposed in which: by using a reproducing head with a numeric aperture (NA) of 0.85 as a condensing lens for converging laser light, with a laser-beam wavelength being set to about 400 nm, a capacity of about 25 GB in the case of a single layer has been achieved, and a capacity of about 50 GB in the case of two layers has been achieved. For example, such an optical disk allows a recording or reproducing operation on or from two signal recording layers from one surface side, and is capable of recording or reproducing information of BS digital broadcasting for about 4 hours.
Referring to FIG. 2, the following description will discuss the structure and a manufacturing method of a two-layer optical disk, which is a conventional optical disk (Japanese Patent Laid-Open Publication No. 2002-092969). FIG. 2 is a cross-sectional view that shows a two-layer optical disk, which is conventional optical disk. The conventional optical disk is constituted by a first substrate 201, a first reflective layer 202, a second substrate 203, a second reflective layer 204 and a cover layer 205 that are successively laminated. On one surface of the first substrate 201, first pits are formed, which have a continuous spiral form as a plane shape, each of which having a concave shape in cross section. The first substrate 201 is designed to have a thickness of about 1.1 mm. This thickness is prepared so as to set the total thickness of the disk to about 1.2 mm. Thus, the disk rigidity is intensified and thickness compatibility with other disks such as CDs and DVDs is achieved. The first reflective layer 202, which reflects lands and recesses of the first pits, is formed on the first pits of the first substrate 201. On the first reflective layer 202, a first signal face 206 is formed, made of pits each having a concave shape, when viewed from the laser light, with a track pitch 207 of about 0.32 μm and a depth of about 70 nm. Here, the first reflective layer 202 reflects laser light that is made incident thereon from the cover layer 205 side. The first reflective layer 202 has a thickness of 50 nm, and is designed to have a reflectance of about 70% with respect to laser light having a wavelength of 400 nm.
With respect to the second substrate 203, second pits having a convex shape are formed on a face on the cover layer 205 side that is the side opposite to the first substrate 201. Moreover, the second reflective layer 204, which reflects lands and recesses of the second pits, is formed on the second pits of the second substrate 203. In the same manner as the first signal face 206 of the first reflective layer 202, on the second reflective layer 204, a second signal face 208 is formed, made of pits each having a convex shape, when viewed from the laser light irradiation side, with a track pitch of about 0.32 μm and a depth of about 70 nm. The lands and recesses of the pits of the second signal face 208 are directed in a direction opposite to the pits of the first signal face 206, and a signal is recorded by these pits. The second substrate 203 is made from a material that virtually transmits laser light. The second reflective layer 204, which is made from the same material as the first reflective layer 202, is formed with a thickness of about 20 nm through a sputtering method. By making the second reflective layer 204 thinner, laser light is partially transmitted, while being partially reflected. The laser light that has been transmitted through the second reflective layer 204 is reflected by the first signal face 206 on the first reflective layer 202, and after having been again transmitted through the second reflective layer 204, is returned to the reproducing head. Here, by setting the thickness of the second reflective layer 204 to 20 nm, the intensity of laser light that has been reflected by the first signal face 206 formed on the first reflective layer 202 and returned to the reproducing head is made equal to the intensity of laser light that has been reflected by the second signal face 208 formed on the second reflective layer 204 and returned to the reproducing head. The cover layer 205, which has a thickness of about 0.1 mm, is made from a material that virtually transmits laser light.
In an attempt to achieve 50 GB by using the above-mentioned structure, supposing that a reproducing head having, for example, a semiconductor laser wavelength of 400 nm with an NA of 0.85 is used, the track pitch TP of signals formed on the first substrate 201 and the second substrate 203 is set to 0.32 μm and the pit length of 2T signals, which forms the shortest pit when 1-7 modulation system is adopted as the signal modulation system, is set to 0.149 μm.
Further, the following description will discuss a manufacturing method for a conventional optical disk.
(a) An injection compression molding process is carried out by using a metal mold stamper on which signals made of pits, each having a concave shape in cross section, and that have a continuous spiral shape on the plane, are formed on one surface. Thus, a first substrate 201 is formed made of resin having first pits that have been formed by transferring the pits of the stamper.
(b) A first reflective layer 202, for example, made of Ag, is formed on the first pits of the first substrate 201 so as to have an even thickness by using a method such as a sputtering method and a vapor deposition method. Thus, a first signal face 206, which reflects the lands and recesses of the first pits, is formed on the first reflective layer 202.
(c) A material such as photo-curing resin is applied to the first reflective layer 202, and a transfer substrate having a transfer signal face of concave-shaped pits is superposed thereon so that second pits, each having a convex-shape, formed by transferring concave-convex shapes of the transfer signal face, are formed on the surface of the photo-curing resin.
(d) The photo-curing resin is photo-cured so that a second substrate 203 having the second pits is formed.
(e) In the same manner as the first reflective layer 202, a second reflective layer 204 is formed on the second pits so as to have an even thickness by using a method such as a sputtering method and a vapor deposition method. Thus, a second signal face 208, which reflects the lands and recesses of the second pits, is formed on the second reflective layer 204. The thickness of the second reflective layer 204 is determined in such a manner that, when reading laser light is made incident on the cover layer side, the quantities of light rays returned to the reproducing head from the respective reflective layers are made equal with one another.
(f) After a sheet made from a material that is virtually transparent to laser light has been formed on the second reflective layer 204 by using photo-curing resin or pressure-sensitive bonding agent, or photo-curing resin has been applied thereon through spin-coating method, this is photo-cured to form a cover layer 205.
An optical disk is manufactured through the above-mentioned respective processes.
Moreover, the following description will discuss a method for reproducing information recorded on the respective signal faces of a conventional two-layer optical disk having the above-mentioned structure.
(a) In the case when the first signal face 206 formed on the first reflective layer 202 is reproduced, for example, the disk is rotated with a desired number of revolutions, and reading laser light is converged by a condensing lens of a reproducing head so that the laser light is focus-controlled as a spot on the first signal face of the optical disk that is rotating at the desired number of revolutions.
(b) Successively, the signal rows are traced by carrying out a known tracking controlling operation so that reflected light is detected from the signal face by a light-receiving element, and read as an analog signal representing a voltage change.
(c) Moreover, in the case when the second signal face 208, formed on the second reflective layer 204 that is the other signal face, is reproduced, in the same manner as the reproducing process from the first signal face 206, reading laser light is converged by a condensing lens of a reproducing head so that the laser light is focus-controlled as a spot on the second signal face of the optical disk that is rotating at the desired number of revolutions.
(d) Successively, the signal rows are traced by carrying out a known tracking controlling operation so that reflected light is detected from the signal face by a light-receiving element, and read as an analog signal representing a voltage change.
In the above-mentioned reproducing process, with respect to the depth of signal pits constituted by lands and recesses formed on the signal face, its optical depth d is virtually made coincident with λ/(4n)(n: the refractive index of a material formed on the signal face) so that the amplitude of the playback signal becomes the greatest. For this reason, with respect to the optical disk reproduction for a read-only memory (ROM), a phase-difference tracking system, which makes the tracking error signal greatest when the playback signal amplitude is the greatest, is adopted in most cases, as a tracking error detection method used for tracking control.
The following description will discuss playback signal characteristics obtained when each of the signal faces of the optical disk is reproduced. In the above-mentioned signal reproduction, the known push-pull tracking error signal TEpp was 0.02. The push-pull tracking error signal TEpp is preliminarily standardized by dividing a push-pull tracking error signal TEpp-org by a sum signal TEsum obtained by voltage-converting the sum of light quantities of light-receiving elements that form the push-pull tracking error signal TEpp-org, so that the reflectance of the disk does not give adverse effects to the signal amplitude. However, the size of this TEpp signal amplitude fails to provide sufficient gain used for tracking control, with the result being that the tracking control is not carried out by receiving adverse effects due to influences of shape changes caused by vibration and deviations in disk manufacturing processes.
Moreover, in the case when the tracking control was carried out by altering the tracking system to the phase-difference tracking system to reproduce signal pits on the first signal face 206, the degree of modulation ((playback signal amplitude of longest pit)/(amount of DC of greatest reflectance of longest pit)) that represents the size of amplitude of the playback signal was 0.45 and the playback signal jitter that represents signal quality was 5.3%. In this case, a known limit equalizer was used for measuring playback signal jitters. Furthermore, in the case when known focusing control was carried out so as to form a spot focused by the reproducing lens of the reproducing head on the second signal face 208 made of signal pits, each having a convex shape when viewed from the laser light irradiation side, formed on the second reflective layer 202, the push-pull tracking error signal TEpp was 0.03. Furthermore, the degree of modulation that represents the size of amplitude of the playback signal RF was 0.40 and the playback signal jitter that represents signal quality was 6.7%, thus, in comparison with the reproducing operation of the first signal face 206, although the push-pull tracking error signal TEpp is prepared as virtually the same signal, the degree of modulation that represents the size of amplitude of the playback signal RF and the playback signal jitter that represents signal quality show that it is not possible to obtain good signal quality due to influences from insufficient transferring of the signal face.
Based upon these facts, it is not possible to carry out tracking control and it is also not possible to obtain a sufficient jitter value that represents playback signal quality, unless a phase-difference tracking system having large power consumption is adopted.
In the conventional optical disk, in most cases, the optical depth d of a signal is set to about λ/(4n), and a phase difference tracking system is adopted as its tracking control system. However, the problem with this phase difference tracking system is that high power consumption is required. In contrast, a push-pull tracking control system, which is another tracking control system, requires lower power consumption in comparison with the phase difference tracking system. However, in the case of the push-pull tracking control system being adopted, due to the fact that the optical depth of pits on the signal face is set to just λ/(4n), it is not possible to obtain a sufficient amplitude in the tracking error signal. Moreover, upon transferring and forming signal pits having the optical depth of λ/(4n) by using photo-curing resin, due to miniaturized pits used for preparing high-density signals, it is not possible to carry out an even transferring process over the entire signal face, resulting in a failure in providing sufficient playback signal quality for reproduction.
The present invention is directed to an optical recording medium that is capable of signal-reproducing in both of the phase difference tracking control system and the push-pull tracking control system as the tracking control system, which has superior reproducing characteristics of recording signals, and to manufacturing method for such an optical recording medium.